1. Field of the Invention
The invention relates generally to wireless communication. In particular, the invention relates to adaptive filters for use in wireless communication systems.
2. Description of Related Art
Wireless communication systems are widely deployed and these systems support the transmission of various types of traffic data, for example, voice, packet data, and other types of data. Communication over a wireless channel can be accomplished using a variety of techniques which facilitate a large number of users in a limited frequency spectrum. These techniques, commonly referred to as multiple access techniques, include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
CDMA offers many advantages over other multiple access techniques used in wireless communication systems such as TDMA and FDMA. For example, CDMA permits the frequency spectrum to be reused multiple times, thereby permitting an increase in system user capacity. Additionally, use of CDMA techniques permits the special problems of the wireless channel to be overcome, for example, by mitigation of the adverse effects of multipath, e.g. fading, while also exploiting the advantages thereof.
In wireless communication systems, a pilot signal is sometimes transmitted from a transmitter unit to a receiver unit and can be used to assist the receiver unit to perform a number of functions. For example, the pilot signal can be used at the receiver unit for synchronization with the timing and frequency of signals transmitted by the transmitter unit, estimation of the quality of the wireless communication channel, coherent demodulation of a data transmission, determination of which specific transmitter unit having the best communication link to the receiver unit, estimation of the highest data rate supportable the wireless channel, and other uses.
Generally, a pilot signal is generated based on a known data pattern and using a known signal processing scheme. For example, in a communication system based on CDMA, a pilot signal is typically a sequence of all zeroes, and the sequence is “covered”, or modulated, with a particular channelization code and “spread”, or modulated, with a known scrambling code or pseudo-noise (PN) sequence.
The wireless communication channel presents challenges to the communication system. One challenge of communication over a wireless channel is commonly referred to as “multipath.” In a wireless communication channel, a transmitted signal may travel multiple different paths, or multipaths, as the signal propagates between a transmitter and a receiver. These multipath signals may be caused, for example, by reflection off of obstacles, such as buildings, bridges, people, and other obstacles as the signal travels from the transmitter to the receiver. These reflected, or multipath, signals are received as multiple instances of the transmitted signal, delayed in time and phase from each other. Because these multipath signals are no longer in phase when they recombine at the receiver, they can result in a lower signal level commonly referred to as a “fade.” In addition, multipath signals change over time as the receiver, the obstacles, or both, move about, causing the paths traveled by the multipath instances of the transmitted signal to change.
A technique used to improve operation of the communication system in a multipath environment is a rake receiver. A rake receiver includes multiple processing “fingers” and each received multipath signal instance of sufficient strength may be assigned to, and processed by, a respective finger processor. Each finger of the rake receiver processes the assigned multipath signal instance, in a manner complementary to that performed at the transmitter unit, to recover a pilot signal and traffic data from the signal received over a multipath communication channel.
The amplitude and phase of the recovered pilot signal will be distorted by, and indicative of, the multipath characteristics of the communication channel, or the channel response. Because the pilot signal and traffic data signal travel through the same communication channel they will, typically, be similarly distorted by the channel response. Knowledge of the amplitude and phase of the pilot signal can be used to align fingers in a rake receiver that are assigned to multipath instances of traffic data signals such that the multiple instances of the transmitted signal may be combined to derive demodulated symbols having improved quality. In addition, knowledge of the amplitude and phase of the pilot signal can be used to reassign fingers as the multipath wireless channel changes over time.
The quality of the recovered pilot signal can impact the performance of the demodulation process, which may in turn impact the overall performance of the communication system. Recovery of the pilot signal typically involves using a pilot filter because a pilot signal is typically degraded by channel noise. In addition, the pilot signal is typically further distorted by fading in the communication channel. Due to these and other reasons, it is challenging to estimate the time-varying response of signals transmitted through the communication channel based upon the received pilot signal. That is, the channel response refers to the variety of changes a signal undergoes from the time it is transmitted until it is received.
Therefore, there is a need in the art for techniques to provide an improved estimate of the time-varying response of a communication channel from a received pilot signal in a wireless communication system. The present invention satisfies this need.